The metal detectors most commonly used, notably for leisure, use the now well-known principle of measuring the phase between an alternating electromagnetic signal transmitted by a coil energized by an electric voltage, and the signal received by another coil close to the first. In this type of apparatus, the two coils are coupled in such a way that the signal received by the receiving coil is zero when no metallic element is placed in the field of the transmitting coil. The frequency of the signal transmitted by the transmitting coil is generally less than 100 khz.
These detectors are customarily composed of 3 parts:                a detection head,        a support stick, which may or may not be telescopic,        an electronic control box.        
Very generally, the detector also comprises an audio headset which allows the electronic control box to transmit an audible signal to the operator when a metallic object is detected.
In this type of equipment the detection head generally takes the form of a disc which essentially contains the detection coils. Accordingly, the excitation signal sent to the transmitting coil as well as the signal received by the receiving coil are conveyed by an electric cable between the detection head and the control box.
The detection head is generally fixed in a removable manner to a telescopic support stick, which comprises a holding handle for the use of the operator who can thus walk along while moving the detection head above the ground.
Generally, the control box contains the major part of the electronic circuits making it possible to generate the excitation signal and to perform the reception and the processing of the signal detected by the receiving coil, the analysis of the signal received making it possible to determine the nature of the corresponding metals. It also contains the batteries necessary for the operation of the assembly.
Depending on the product considered, the control box can be positioned on the support stick, facing the user, so as to allow him to access this box with the hand not mobilized by holding the equipment. The control box furthermore comprises, usually, a loudspeaker and an audio output making it possible to connect a headset.
With the aim of decreasing the weight carried by the support stick and of increasing the handleability of the detection head, certain equipment offer a carrying bag in which the control box can be housed and carried slung over the shoulder by the operator. In this way, the support stick is advantageously made lighter and more handleable. Conversely, the necessary linking of the box with the detection head compels the installation of a longer electric linking cable, then intended to hang by the side of the operator and which is at risk of catching on vegetation in the case where the search for metal is performed in an area of dense and bushy vegetation.
More generally, although apparently being the simplest to implement, the use of electric cables to link various elements of the detector and enable them to communicate with one another turns out to be to blame for often not-inconsiderable drawbacks to use.
Thus the cable linking the detection head to the control box is a bulky and fragile element which gives rise to multiple inconveniences during use, folding up of the stick, stowing of the apparatus and generally during fitting and dismantling operations.
Likewise, when the user wishes to change detection-disc diameter to accommodate the terrain or the targets to be detected, he must then dismantle the connector which links the cable to the electronic control box and undo the ties which hold the cable to the support stick, this being lengthy and irksome.
Likewise again, in the case where the electronic box is worn on a belt or slung over the shoulder, the cable remains hanging between the top of the telescopic stick, along which it runs as far as the detection head and the box which is then not fixed to the stick. It can therefore get caught on multiple obstacles, making for example searching difficult in a bushy area or forest. Furthermore, when the user has to dig up the ground to extract a metallic target, he is still linked to the stick by the cable, thereby limiting him in his movements.
Likewise again, detectors being by nature perpetually in motion in often difficult outdoor conditions, the cable and its connection facility are severely tested and regularly break down. Repair then requires a return to the workshop. To limit these breakdowns, one is constrained furthermore to use particularly strong, and therefore expensive, connection elements and to take particular care over the installation of these connecting elements during the manufacture of the detector, thereby giving rise to significant labour costs in so far as this connection facility over which care was taken is fitted manually.
Finally, it should be noted that, from the electronic standpoint, the cable is not an innocuous element. It may even be a nuisance component in the detection-signal-processing chain, all the more so since it is usually connected to the transmission and reception oscillating circuits. Noise or couplings may then occur between these various conductors. The cable may also, in the guise of metallic element, be detected by the detection head, notably if it is placed too near the latter. These problems add constraints that must be taken into account by the manufacturer during development.
U.S. Pat. No. 5,696,490 describes a detector using very high frequencies (VHF) to detect metal, and proposes that the problems generated by the cable be solved by replacing the wired link with an infrared-wave wireless link.
However, in the detector presented, the reception and display system described as being positioned at the bottom of the detection stick, in front of the disc, this not affording any benefit in terms of simplification or integration. Moreover, in the device presented, several leads are nevertheless necessary for linking inter alia the coils to the receiver box.
U.S. Pat. No. 5,138,262 describes for its part a detachable system for housing batteries that can be worn on a belt, making it possible to lighten the detector and to reduce the user effort. However, even if this system somewhat lightens the load supported at the end of the arm by the user and thus somewhat eases the handling of the detector, the user remains despite everything, in such a configuration, linked and attached to the detector by a bulky cable which limits him in these movements notably when he has to bend down to dig or enter bushy areas.